Remove TotalCountChanged.

Merge it into existing `IChildIndexProvider.ChildIndexChanged` event.

src/Avalonia.Base/LogicalTree/ChildIndexChangedEventArgs.cs

@@ -1,26 +1,59 @@
-#nullable enable
+using System;
-using System;
+
+#nullable enable
 
 namespace Avalonia.LogicalTree
 {
     /// <summary>
-    /// Event args for <see cref="IChildIndexProvider.ChildIndexChanged"/> event.
+    /// Describes the action that caused a <see cref="IChildIndexProvider.ChildIndexChanged"/> event.
     /// </summary>
-    public class ChildIndexChangedEventArgs : EventArgs
+    public enum ChildIndexChangedAction
     {
-        public static new ChildIndexChangedEventArgs Empty { get; } = new ChildIndexChangedEventArgs();
+        /// <summary>
+        /// The index of a single child changed.
+        /// </summary>
+        ChildIndexChanged,
 
-        private ChildIndexChangedEventArgs()
+        /// <summary>
-        {
+        /// The index of multiple children changed and all children should be re-evaluated.
-            Index = -1;
+        /// </summary>
-        }
+        ChildIndexesReset,
+
+        /// <summary>
+        /// The total number of children changed.
+        /// </summary>
+        TotalCountChanged,
+    }
 
+    /// <summary>
+    /// Event args for <see cref="IChildIndexProvider.ChildIndexChanged"/> event.
+    /// </summary>
+    public class ChildIndexChangedEventArgs : EventArgs
+    {
+        /// <summary>
+        /// Initializes a new instance of the <see cref="ChildIndexChangedEventArgs"/> class with
+        /// an action of <see cref="ChildIndexChangedAction.ChildIndexChanged"/>.
+        /// </summary>
+        /// <param name="child">The child whose index was changed.</param>
+        /// <param name="index">The new index of the child.</param>
         public ChildIndexChangedEventArgs(ILogical child, int index)
         {
+            Action = ChildIndexChangedAction.ChildIndexChanged;
             Child = child;
             Index = index;
         }
 
+        private ChildIndexChangedEventArgs(ChildIndexChangedAction action)
+        {
+            Action = action;
+            Index = -1;
+        }
+
+        /// <summary>
+        /// Gets the type of change action that ocurred on the list control.
+        /// </summary>
+        public ChildIndexChangedAction Action { get; }
+
         /// <summary>
         /// Gets the logical child whose index was changed or null if all children should be re-evaluated.
         /// </summary>
@@ -30,5 +63,17 @@ namespace Avalonia.LogicalTree
         /// Gets the new index of <see cref="Child"/> or -1 if all children should be re-evaluated.
         /// </summary>
         public int Index { get; }
+
+        /// <summary>
+        /// Gets an instance of the <see cref="ChildIndexChangedEventArgs"/> with an action of
+        /// <see cref="ChildIndexChangedAction.ChildIndexesReset"/>.
+        /// </summary>
+        public static ChildIndexChangedEventArgs ChildIndexesReset { get; } = new(ChildIndexChangedAction.ChildIndexesReset);
+
+        /// <summary>
+        /// Gets an instance of the <see cref="ChildIndexChangedEventArgs"/> with an action of
+        /// <see cref="ChildIndexChangedAction.TotalCountChanged"/>.
+        /// </summary>
+        public static ChildIndexChangedEventArgs TotalCountChanged { get; } = new(ChildIndexChangedAction.TotalCountChanged);
     }
 }

src/Avalonia.Base/LogicalTree/IChildIndexProvider.cs

@@ -28,10 +28,5 @@ namespace Avalonia.LogicalTree
         /// Notifies subscriber when a child's index was changed.
         /// </summary>
         event EventHandler<ChildIndexChangedEventArgs>? ChildIndexChanged;
-
-        /// <summary>
-        /// Notifies subscriber when the total child count changes.
-        /// </summary>
-        event EventHandler<EventArgs>? TotalCountChanged;
     }
 }

src/Avalonia.Base/Styling/Activators/NthChildActivator.cs

@@ -37,7 +37,6 @@ namespace Avalonia.Styling.Activators
         protected override void Initialize()
         {
             _provider.ChildIndexChanged += ChildIndexChanged;
-            _provider.TotalCountChanged += TotalCountChanged;
         }
 
         protected override void Deinitialize()
@@ -48,9 +47,11 @@ namespace Avalonia.Styling.Activators
         private void ChildIndexChanged(object? sender, ChildIndexChangedEventArgs e)
         {
             // Run matching again if:
-            // 1. e.Child is null, when all children indices were changed.
+            // 1. Subscribed child index was changed
-            // 2. Subscribed child index was changed.
+            // 2. Child indexes were reset
-            if (e.Child is null || e.Child == _control)
+            // 3. We're a reversed (nth-last-child) selector and total count has changed
+            if ((e.Child == _control || e.Action == ChildIndexChangedAction.ChildIndexesReset) ||
+                (_reversed && e.Action == ChildIndexChangedAction.TotalCountChanged))
             {
                 // We're using the _index field to pass the index of the child to EvaluateIsActive
                 // *only* when the active state is re-evaluated via this event handler. The docs

src/Avalonia.Controls.DataGrid/Primitives/DataGridCellsPresenter.cs

@@ -336,7 +336,7 @@ namespace Avalonia.Controls.Primitives
 
         internal void InvalidateChildIndex()
         {
-            _childIndexChanged?.Invoke(this, ChildIndexChangedEventArgs.Empty);
+            _childIndexChanged?.Invoke(this, ChildIndexChangedEventArgs.ChildIndexesReset);
         }
 
         private bool ShouldDisplayCell(DataGridColumn column, double frozenLeftEdge, double scrollingLeftEdge)

src/Avalonia.Controls.DataGrid/Primitives/DataGridColumnHeadersPresenter.cs

@@ -423,7 +423,7 @@ namespace Avalonia.Controls.Primitives
 
         internal void InvalidateChildIndex()
         {
-            _childIndexChanged?.Invoke(this, ChildIndexChangedEventArgs.Empty);
+            _childIndexChanged?.Invoke(this, ChildIndexChangedEventArgs.ChildIndexesReset);
         }
     }
 }

src/Avalonia.Controls/ItemsControl.cs

@@ -101,7 +101,6 @@ namespace Avalonia.Controls
         private int _itemCount;
         private ItemContainerGenerator? _itemContainerGenerator;
         private EventHandler<ChildIndexChangedEventArgs>? _childIndexChanged;
-        private EventHandler<EventArgs>? _totalCountChanged;
         private IDataTemplate? _displayMemberItemTemplate;
         private ScrollViewer? _scrollViewer;
         private ItemsPresenter? _itemsPresenter;
@@ -218,12 +217,6 @@ namespace Avalonia.Controls
             remove => _childIndexChanged -= value;
         }
 
-        event EventHandler<EventArgs>? IChildIndexProvider.TotalCountChanged
-        {
-            add => _totalCountChanged += value;
-            remove => _totalCountChanged -= value;
-        }
-
         /// <inheritdoc />
         public event EventHandler<RoutedEventArgs> HorizontalSnapPointsChanged
         {
@@ -500,7 +493,7 @@ namespace Avalonia.Controls
             else if (change.Property == ItemCountProperty)
             {
                 UpdatePseudoClasses(change.GetNewValue<int>());
-                _totalCountChanged?.Invoke(this, EventArgs.Empty);
+                _childIndexChanged?.Invoke(this, ChildIndexChangedEventArgs.TotalCountChanged);
             }
             else if (change.Property == ItemContainerThemeProperty && _itemContainerGenerator is not null)
             {
@@ -585,7 +578,7 @@ namespace Avalonia.Controls
         internal void RegisterItemsPresenter(ItemsPresenter presenter)
         {
             Presenter = presenter;
-            _childIndexChanged?.Invoke(this, ChildIndexChangedEventArgs.Empty);
+            _childIndexChanged?.Invoke(this, ChildIndexChangedEventArgs.ChildIndexesReset);
         }
 
         internal void PrepareItemContainer(Control container, object? item, int index)

src/Avalonia.Controls/Panel.cs

@@ -35,7 +35,6 @@ namespace Avalonia.Controls
         }
 
         private EventHandler<ChildIndexChangedEventArgs>? _childIndexChanged;
-        private EventHandler<EventArgs>? _totalCountChanged;
 
         /// <summary>
         /// Initializes a new instance of the <see cref="Panel"/> class.
@@ -67,12 +66,6 @@ namespace Avalonia.Controls
             remove => _childIndexChanged -= value;
         }
 
-        event EventHandler<EventArgs>? IChildIndexProvider.TotalCountChanged
-        {
-            add => _totalCountChanged += value;
-            remove => _totalCountChanged -= value;
-        }
-
         /// <summary>
         /// Renders the visual to a <see cref="DrawingContext"/>.
         /// </summary>
@@ -161,7 +154,7 @@ namespace Avalonia.Controls
                     throw new NotSupportedException();
             }
 
-            _childIndexChanged?.Invoke(this, ChildIndexChangedEventArgs.Empty);
+            _childIndexChanged?.Invoke(this, ChildIndexChangedEventArgs.ChildIndexesReset);
             InvalidateMeasureOnChildrenChanged();
         }
 
@@ -173,7 +166,7 @@ namespace Avalonia.Controls
         private void ChildrenPropertyChanged(object? sender, PropertyChangedEventArgs e)
         {
             if (e.PropertyName == nameof(Children.Count) || e.PropertyName is null)
-                _totalCountChanged?.Invoke(this, EventArgs.Empty);
+                _childIndexChanged?.Invoke(this, ChildIndexChangedEventArgs.TotalCountChanged);
         }
 
         private static void AffectsParentArrangeInvalidate<TPanel>(AvaloniaPropertyChangedEventArgs e)